A lot of long-distance American tractor trailers deploy fold-out cones on their back doors to improve their aerodynamics while driving. They fold up for low speed driving and loading.

Also, fold-out flaps under the trailer for the same reason.

Is it something with state by state safety regulations, like prohibitions on protruding structures that aren't part of the main body? Or differently-defined overall length restrictions? Does a given long haul truck have them deployed in the middle of the country, but then they're removed before getting to the coastal states?

There's an interesting mis-alignment of incentives. It comes down to who owns the truck vs the trailer.

* Truck owner pays for fuel.

* Trailer owner pays for the flaps.

So, unless you own both the truck and trailer, there's little incentive for trailer owners to install them.

In vehicles, there's a lot of additional, interesting factors at play - like engine efficiency.